Abstract:

A method is presented for producing a silicon solar cell with a
back-etched emitter preferably with a selective emitter and a
corresponding solar cell. According to one aspect, the method comprises
the following method steps: producing a two-dimensionally extending
emitter at an emitter surface of a solar cell substrate; applying an
etching barrier onto first partial zones of the emitter surface; etching
the emitter surface in second partial zones of the emitter surface not
covered by the etching barrier; removing the etching barrier; and
producing metal contacts at the first partial zones. During the method,
especially during the etching of the emitter surface in the second
partial zones, a porous silicon layer is advantageously produced, which
is then oxidised. This oxidised porous silicon layer can subsequently be
etched away together with any phosphorus glass that may be present. The
method makes use of conventional screen-printing and etching technologies
and is thus compatible with current industrial production plants.

Claims:

1-14. (canceled)

15. A method for producing a silicon solar cell, wherein the method
comprises in the following order:producing a two-dimensionally extending
emitter at an emitter surface of a solar cell substrate;producing a
porous silicon layer at the emitter surface; andback-etching the layer of
porous silicon.

16. The method according to claim 15, further comprising:oxidising the
porous silicon layer.

17. The method according to claim 16, further comprising etching of the
porous icon layer.

18. The method according to claim 17, wherein the etching of the oxidised
porous silicon layer is carried out after the removal of the etching
barrier.

19. The method according to claim 15, wherein the thickness of the
produced porous silicon layer is detected optically.

20. The method according to claim 15, wherein at least one of the steps of
etching of the emitter surface, producing of the porous silicon layer and
oxidising of the porous silicon layer is carried out with a liquid
solution.

21. The method according to claim 15, wherein the production of the
two-dimensionally extending emitter is carried out by means of a
POCl3 gas-phase diffusion and wherein the phosphorus gas thereby
arising is not removed before the application of the etching barrier.

22. The method according to claim 15, wherein the etching barrier is
applied with the aid of a paste containing plastic.

23. The method according to claim 15, wherein the etching barrier is
applied by means of screen-printing.

24. The method according to claim 15, wherein the metal contacts are
applied by means of screen-printing.

25. A method for producing a silicon solar cell with a selective emitter,
wherein the method comprises in the following order:producing a
two-dimensionally extending emitter at an emitter surface of a solar cell
substrate;applying an etching barrier onto first partial zones of the
emitter surface;etching the emitter surface in second partial zones of
the emitter surface not covered by the etching barrier;removing the
etching barrier; andproducing metal contacts at the first partial zones.

26. The method according to claim 25, further comprising after the
deposition of the etching barrier:producing a porous silicon layer at the
second partial zones of the emitter surface not covered by the etching
barrier.

27. The method according to claim 26, further comprising:oxidising the
porous silicon layer.

28. The method according to claim 26, further comprising etching of the
porous silicon layer.

29. The method according to claim 27, further comprising etching of the
porous silicon layer.

30. The method according to claim 28, wherein the etching of the oxidised
porous silicon layer is carried out after the removal of the etching
barrier.

31. The method according to claim 29, wherein the etching of the oxidised
porous silicon layer is carried out after the removal of the etching
barrier.

32. The method according to claim 26, wherein the thickness of the
produced porous silicon layer is detected optically.

33. The method according to claim 25, wherein at least one of the steps of
etching of the emitter surface, producing of the porous silicon layer and
oxidising of the porous silicon layer is carried out with a liquid
solution.

34. The method according to claim 25, wherein the production of the
two-dimensionally extending emitter is carried out by means of a
POCl3 gas-phase diffusion and wherein the phosphorus gas thereby
arising is not removed before the application of the etching barrier.

35. The method according to claim 25, wherein the etching barrier is
applied with the aid of a paste containing plastic.

36. The method according to claim 25, wherein the etching barrier is
applied by means of screen-printing.

37. The method according to claim 25, wherein the metal contacts are
applied by means of screen-printing.

38. A silicon solar cell with a selective emitter, wherein the silicon
solar cell comprises:a solar cell substrate with a two-dimensionally
extending emitter at an emitter surface thereof;a dielectric
layer;emitter metal contacts at the emitter surface;wherein the
two-dimensionally extending emitter has a higher surface doping
concentration in first partial zones than in adjacent second partial
zones;wherein the solar cell substrate has a greater thickness in the
first partial zones than in the second partial zones; andwherein the
dielectric layer essentially covers the whole emitter surface and is
arranged locally between the emitter metal contacts and the solar cell
substrate.

39. The solar cell according to claim 38, wherein the metal contacts at
least locally penetrate the dielectric layer.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a national phase entry under 35 U.S.C. §371
of International Application No. PCT/EP2008/059647, filed Jul. 23, 2008
in English, which claims the benefit of the filing date of German Patent
Application No. 10 2007 035 068.8 filed Jul. 26, 2007 and of German
Patent Application No. 10 2007 062 750.7 filed Dec. 27, 2007. The
disclosures of said applications are hereby incorporated herein by
reference.

BACKGROUND OF THE INVENTION

[0002]The present invention relates to a method for producing a silicon
solar cell with a back-etched, preferably selective emitter as well as a
corresponding solar cell.

[0003]It is known that emitters produced at a surface of a solar cell
often exhibit, for production-related reasons, a high doping
concentration directly at the surface. This high doping concentration can
lead to recombination losses, especially with respect to charge carrier
pairs generated close to the surface.

[0004]It may therefore be desirable to make available a production method
for a solar cell, wherein the doping concentration at the surface of the
emitter can be reduced in a technologically straightforward manner.

[0005]For the most part, the solar cells currently manufactured
industrially are produced based on silicon, especially crystalline
silicon. The vast majority of these solar cells are provided with a
full-area homogeneous emitter layer at the front-side surface and/or at
the rear-side surface of the solar cell substrate. The metal contacts are
produced by means of thick-film pastes in the screen-printing process in
the case of many of the silicon solar cells industrially produced
nowadays. For this purpose, a metal-particle-containing paste is printed
locally onto the front-side emitter and then fired into the emitter, in
order to form a good electrical contact with the emitter layer.

[0006]It is known here that it may be necessary to provide the emitter
layer, at least in the zones contacted by the metal contacts, with a high
doping concentration in the region of the emitter surface in order to
obtain a good ohmic contact.

[0007]A characteristic parameter for assessing the quality of the emitter,
i.e. the doping concentration integrated over the cross-section of the
emitter layer, is the so-called sheet resistance. The greater the sheet
resistance, the smaller the doping concentration inside the emitter layer
and the smaller, as a rule, the doping concentration at the surface of
the emitter layer. It has been found that, with conventionally produced
emitters, a maximum sheet resistance of emitters capable of being
contacted with screen-printing metallisation techniques typically lies in
the range from 50-60 ohms per square. Emitter layers with higher sheet
resistances and thus fundamentally lower doping usually can no longer be
contacted reliably by means of thick-film pastes.

[0008]When using industrially advantageous screen-printing metallisation
techniques, it is therefore necessary to make available emitter layers
with a high surface doping concentration in the region of the metal
contacts. On the other hand, however, it is known that such a high
surface doping concentration can be accompanied by heavy recombination
losses at the surface of the solar cell. In particular, charge carrier
pairs which are produced by high-frequency (blue or UV) light very close
to the front-side solar cell surface recombine rapidly inside this
strongly doped emitter layer and can therefore no longer contribute to
the solar cell current. This can reduce the IQE (internal quantum
efficiency) in the high-frequency light spectrum and thus the total
current supplied by the solar cell, which ultimately reduces the
efficiency of the solar cell. An additional effect of a high surface
doping concentration may be a so-called band gap narrowing, which can
lead to a reduced open circuit voltage. Attempts to meet these
contradictory requirements for good contactability on the one hand and a
high IQE on the other hand led to the concept of the so-called selective
emitter. In the case of the latter, the emitter zones directly beneath
the metal contacts are strongly doped locally, whereas the zones lying in
between have a much lower doping concentration.

[0009]Several methods for producing selective emitter structures have
already been developed and tested, mainly on the laboratory scale. In one
approach, a selective emitter structure can be produced by two separate
diffusion processes in two separate process steps using a local masking
layer, for which dielectric layers are often used. Here, however, there
is the need for a plurality of high-temperature diffusion processes, and
this can increase the production cost significantly. Alternatively, a
selective emitter structure can be produced by local etching of an
emitter layer previously produced homogeneously.

[0010]However, such production methods are often not compatible with the
other process steps currently used industrially, such as for example the
screen-printing metallisation. Furthermore, problems can occur in the
sense that the doping concentration is locally inhomogeneous in the
individual emitter zones due to a non-uniform etching process.

[0011]Previous approaches at producing silicon solar cells with a
selective emitter using two diffusion processes have usually been
technically expensive and scarcely able to be implemented industrially on
account of their high cost. On the other hand, the production of a
selective emitter structure by local etching of the emitter has for the
most part only been achieved in the laboratory hitherto, methods chiefly
having been tested in which etching of the emitter has been carried out
after a metallisation of the solar cells. These production methods have
usually led to a considerable decline in the efficiency of the solar
cells or were scarcely able to be implemented on an industrial scale.

SUMMARY OF THE INVENTION

[0012]It can therefore be regarded as a problem of the present invention
to propose a method for producing a silicon solar cell, especially with a
selective emitter, wherein the aforementioned problems of the prior art,
in particular, can be at least partially overcome. In particular, a
production method for a silicon solar cell may be desired, wherein an
emitter with a low doping concentration at the surface can be obtained.
Furthermore, a production method for a silicon solar cell with a
selective emitter may be desired that is compatible with other
conventional, industrially established production steps, is
cost-effective and permits the production of solar cells with a high
efficiency. Furthermore, a need for a corresponding solar cell may exist.

[0013]These problems and requirements can be met by a method and a solar
cell according to the independent claims. Advantageous embodiments are
indicated in the dependent claims.

[0014]According to a first aspect of the present invention, a method for
the production of a silicon solar cell with a back-etched emitter is
presented, wherein the method comprises in the following order: producing
a two-dimensionally extending emitter at an emitter surface of a solar
cell substrate; producing a layer of porous silicon at the emitter
surface; back-etching the layer of porous silicon.

[0015]The indicated method according to the invention, according to the
first aspect, can be regarded as being based on the idea that a layer of
porous silicon is produced in a emitter layer and is then back-etched, in
order in this way to remove the near-surface, strongly doped zones of the
emitter layer. As is stated in greater detail below with respect to
preferred embodiments, such back-etching of a porous layer can be carried
out in a technologically straightforward manner.

[0016]According to a second aspect of the present invention, a method for
producing a silicon solar cell with a selective emitter is presented,
wherein the method comprises the following steps: producing a
two-dimensionally extending emitter at an emitter surface of a solar cell
substrate; applying an etching barrier onto first partial zones of the
emitter surface; etching the emitter surface in second partial zones of
the emitter surface not covered by the etching barrier; removing the
etching barrier; and producing metal contacts at the first partial zones.
The method steps are preferably carried out in the order indicated.

[0017]The indicated method according to the invention, according to the
second aspect, can be regarded as being based on the idea that, in the
first place, an emitter is produced on at least one surface of a solar
cell substrate with a homogeneous doping concentration, which is high
enough for it to be suitable, for example, for contacting in the
screen-printing process. First partial zones of the emitter surface are
protected by an etching barrier, preferably directly after this
production of the two-dimensionally extending emitter, i.e. preferably
before the deposition of a dielectric layer, for example as an
antireflection or passivation layer, and also preferably before the
application of the metal contacts. The unprotected zones of the emitter
surface are then etched and the thickness of the emitter in these zones
is thus reduced, so that an emitter with an increased sheet resistance
arises in these second partial zones. The etching barrier is then removed
and the solar cell can be further processed in a conventional manner,
i.e. a dielectric layer can for example be deposited at the front-side
surface as an antireflection or passivation layer, and then metal
contacts can then be applied over the latter, for example in the
screen-printing process.

[0018]The presented method according to the first or second aspect can
bring a number of advantages. A cost-effective method that can readily be
implemented industrially can be made available for producing a full-area
or partial-area back-etched, preferably selective emitter structure. To
advantage, technologies can be used for the individual method steps that
have already been used and tried and tested on an industrial scale. For
example, an etching-resistant lacquer or resist can be applied by means
of screen-printing as an etching barrier and the subsequent etching can
be carried out with the aid of conventional wet-chemical etching
processes. The method advantageously dispenses with the use of
cost-intensive vacuum technologies.

[0019]Since use is preferably made solely of technologies which have in
any case long been in use in solar cell production, the technological
risk with the implementation of the method can be kept low. Chemicals
that can be used for the etching step are already used in the production
of solar cells. Screen-printing lacquers which can be used as an etching
barrier are already used in the industrial manufacture of printed circuit
boards. For the implementation of the method, therefore, technologies,
employed media and consumables as well as their methods of disposal are
known, fully developed and already in use.

[0020]Since the method can in particular also be used in a way in which
only a few, easily controllable method steps are added to a conventional
processing sequence for the production of silicon solar cells, the method
can in particular be easily integrated into existing production plants by
the installation of one or more additional modules.

[0021]Further details, possible advantages and preferred embodiments of
the method according to the invention are explained below.

[0022]The method can be used for the production of any silicon solar cell.
For example, solar cells can be produced based on mono-crystalline or
multi-crystalline silicon wafers or alternatively also based on a
crystalline or amorphous silicon thin layer.

[0023]A selective emitter is formed at a surface of the solar cell
substrate referred to hereinafter as "emitter surface". To advantage, the
"emitter surface" can be the front-side surface of the solar cell
substrate pointing towards the sun in use. Alternatively or additionally,
an emitter can also be formed at a rear-side surface. A selective emitter
is understood here to mean a doped semiconductor layer of a conduction
type (for example n-type) opposite to the conduction type of the basis
substrate (for example p-type), wherein the doping concentration varies
greatly locally over the emitter area. For example, the emitter can be
strongly doped before the back-etching, i.e. in the variant of the
selective emitter first partial zones of the emitter at which front
metallisation is subsequently to be disposed can be strongly doped, with
a high surface doping concentration of, for example, more than
5×1019 cm-3, which leads to a sheet resistance of, for
example, less than 60 ohms per square, preferably less than 50 ohms per
square, and more preferably less than 40 ohms per square in these emitter
zones, whereas, in the variant of the selective emitter, other emitter
zones lying in between can be doped more weakly, for example with a
surface doping concentration of less than 1×1019 cm-3,
which leads to a sheet resistance of, for example, more than 60 ohms per
square, preferably more than 70 ohms per square, and more preferably more
than 80 ohms per square. The strongly doped zones can subsequently be
contacted electrically very well with metal contacts, whereas the weakly
doped zones can exhibit a higher IQE and lower emitter saturation
current.

[0024]Details of the invention according to the second aspect described
above will be explained in greater detail below, wherein the remarks can
of course also be transferred analogously to the corresponding features
of the invention according to the first aspect.

[0025]As the first main method step of the method presented, a
two-dimensionally extending emitter is produced at the emitter surface of
a solar cell substrate. Any methods can be used for this purpose. For
example, the two-dimensionally extending emitter can, as described below
in greater detail, be diffused into the surface of the solar cell
substrate by means of a POCl3 gas-phase diffusion by diffusing
phosphorus from a hot gas phase. Any other technologies can however also
be used, such as for example diffusion from a, for example, printed solid
doping source, deposition of an additional separate emitter layer,
spray-on or spin-on of phosphorus-containing substances, implantation of
doping agents into the surface of the solar cell substrate, etc. The
parameters for producing the two-dimensionally extending emitter are
selected in such a way that an emitter sheet resistance of less than 60
ohms per square is preferably established, preferably less than 50 ohms
per square and more preferably less than 40 ohms per square.

[0026]An etching barrier is then applied onto first partial zones of the
front-side surface of the solar cell substrate. The most varied
technologies can be used for this purpose. Use is preferably made of
technologies which are easy to implement industrially, such as for
example the printing of a thick-film paste by means of screen-printing,
the local spraying-on of a curable solution by means of an inkjet
process, aerosol printing, vapour deposition through a mask, etc.

[0027]As an etching barrier, a material is selected which is such that it
is not attacked during the subsequent etching step by the etching medium
used, so that the etching barrier can protect the underlying first
partial zones of the emitter surface against the etching medium.

[0028]The zones of the emitter surface of the solar cell substrate not
protected by the etching barrier are then etched with the aid of an
etching medium. As an etching medium, various etching fluids can be used
which can attack and dissolve the material of the solar cell substrate at
its front side. Since this material is as a rule silicon or, e.g. after
an additional optional oxidation step, silicon oxide, consideration is
given for example to gases or solutions which contain for example
hydrofluoric acid (HF) and/or nitric acid (HNO3).

[0029]The emitter surface is preferably etched down in the second partial
zones to an extent such that, in the remaining emitter layer, a desired
high sheet resistance of, for example, more than 60 ohms per square,
preferably more than 70 ohms per square and more preferably more than 80
ohms per square is established with a surface concentration of the doping
agent, such as for example phosphorus, that is reduced by wet-chemical
etching. The sheet resistance of the remaining emitter layer can be
checked during the etching process, in that the thickness of the layer
already etched down, for example, is observed optically, or the etching
process can be interrupted briefly in order to measure the layer
resistance, for example by means of a conventional 4-point measurement.
In this way, the etching process can be terminated when a predetermined
limiting value for the sheet resistance is reached. Alternatively, the
duration of the etching process up to reaching a specific desired sheet
resistance can be ascertained by preliminary tests.

[0030]After the etching process, the etching barrier is again removed from
the substrate surface. This preferably takes place chemically, for
example with the aid of a solution which attacks and dissolves the
etching barrier.

[0031]Optionally, a further etching process can follow, in which a
phosphorus glass produced in a POCl3 diffusion, for example, can be
etched away, wherein in the same step the second partial zones already
etched in the preceding etching step can be further etched or an oxide
optionally produced there can be etched away. The sheet resistance
ultimately established in the second partial zones can thus be influenced
by the first etching process described above as well as by this optional
second etching process.

[0032]Optionally, further etching steps can follow, such as for example
the preparing of a dielectric layer as an anti-reflection and/or
passivation layer on the emitter surface of the solar cell substrate.

[0033]Metal contacts are then produced at the first partial zones, which
have been protected temporarily by the etching barrier against etching of
the emitter layer in the preceding course of the process. Due to the high
surface doping concentration, i.e. the low sheet resistance, in these
first partial zones, a good ohmic contact can be achieved between the
metal contacts and the silicon solar cell substrate. The metal contacts
can be produced using any technologies. Technologies which can easily be
implemented industrially, such as for example the screen-printing of a
metal-particle-containing thick-film paste, are however preferred.

[0034]According to the first aspect and a preferred embodiment of the
second aspect, the method comprises the additional step of producing a
porous silicon layer. This method step is carried out, in the embodiment
of the second aspect, after the deposition of the etching barrier at the
second partial zones of the emitter surface of the solar cell substrate
not covered by the etching barrier. This method step can preferably be
carried out simultaneously with the method step of etching the emitter
surface in the second partial zones not covered by the etching barrier.
In other words, instead of etching the emitter surface area-wide in the
zones unprotected by the etching barrier, an etching method is selected
which leads to the formation of an at least partially porous silicon
layer. This can be achieved by the correct selection of a suitable
etching solution and suitable etching boundary conditions, such as for
example a suitable temperature, a suitable etching duration, etc. The
porous silicon layer can be produced with good spatial homogeneity and
the thickness of the resultant silicon layer can be influenced by a
suitable selection of the process parameters.

[0035]According to a further embodiment, the previously produced porous
silicon layer is then oxidised. For this purpose, it can be subjected for
example to an oxidising medium, such as for example an acid or an
ozone-containing, wet-chemical bath or another ozone-containing or
ozone-producing source. To advantage, the etching barrier should in this
case also be resistant to this oxidising medium.

[0036]The process parameters during the oxidising of the porous silicon
layer, for example process duration, process temperature, oxidation
capacity of the medium used, etc., can be selected such that the whole
porous layer is oxidised.

[0037]According to another embodiment, the porous silicon layer,
previously oxidised as the case may be, can then be etched in a further
method step and thus be removed. Due to the fact that the surface of the
emitter layer is not simply etched area-wide in a single etching step,
but that a porous layer is first produced which is subsequently oxidised
and then etched away, a more homogeneous etching results can be achieved.
The porous silicon layer can also be alkaline etched without having been
oxidised beforehand.

[0038]According to yet another embodiment, the etching of the oxidised
porous silicon layer is carried out after the removal of the etching
barrier. To advantage, use can be made of the fact that the oxidised
porous silicon layer can be attacked by etching media which cannot or can
scarcely attack pure, non-oxidised silicon. According to this embodiment,
therefore, etching can first be carried out in the second partial zones
unprotected by the etching barrier and a porous silicon layer can be
produced and then oxidised; the etching barrier can subsequently be
removed; the porous silicon layer can then be removed in a further
etching process, whereby, with a suitable selection of the etching
medium, the first partial zones previously protected by the etching
barrier cannot or can scarcely be attacked in this process. To advantage,
a phosphorus glass produced during the production of the
two-dimensionally extending emitter can also be jointly removed in this
additional etching step.

[0039]According to another embodiment, the thickness of the produced
porous silicon layer can be detected optically. The porous silicon layer
has a refractive index that is different from solid silicon, so that
interference effects can occur with thin porous silicon layers. Depending
on the thickness of the porous silicon layer, the latter can, similar to
an antireflection layer, appear in different colours. The thickness of
the produced porous silicon layer can thus already be estimated during
the etching process on the basis of the colour of the porous silicon
layer. Since the porous silicon layer is preferably completely removed in
a following etching step, as a result of which the thickness of the
emitter layer remaining beneath in the solar cell substrate is reduced,
it is thus possible indirectly to deduce optically how high the sheet
resistance of the remaining emitter layer will be after removal of the
porous silicon layer. Alternatively, the thickness of the produced porous
silicon layer could also be determined ellipsometrically.

[0040]According to another embodiment, the step of etching the emitter
surface, the production of the porous silicon layer and/or the oxidation
of the porous silicon layer is carried out in a liquid solution. Acid
solutions, for example, can be used. For example, use can be made of
solutions of acids which attack silicon or silicon oxide, such as for
example HF, HNO3, H2SO4 or combinations thereof. The use
of liquid etching solutions enables, amongst other things, very
homogeneous etching with a high etching capacity and/or etching rate.

[0041]According to another embodiment, the production of the
two-dimensionally extending emitter is carried out by means of a
POCl3 gas-phase diffusion or by spray-on or spin-on and the
phosphorus glass thereby arising is not removed before the application of
the etching barrier. The POCl3 gas-phase diffusion can readily be
implemented industrially and finds widespread use. The phosphorus glass
arising in the diffusion usually has to be removed again from the emitter
surface after the diffusion, before, for example, further antireflection
or passivation layers are deposited. In the method presented, this
etching away of the phosphorus glass does not however have to take place
directly after the diffusion, when it would represent an additional
method step, but can be carried out simultaneously with one of the
subsequent etching steps, for example the etching step for removing the
porous silicon layer after the removal of the etching barrier. In this
way, the processing outlay and the associated costs can be reduced.

[0042]According to another embodiment, the etching barrier is applied with
the aid of a paste containing plastic. Such a paste can be highly viscous
(thick-flowing), so that it can be printed locally, for example by means
of conventional screen-printing technology, onto the first partial zones
of the emitter surface to be protected. Alternatively, the paste can be
low-viscous (fluid), so that it can be sprayed on locally for example in
the inkjet process. The initially viscous paste can then be hardened, for
example by heat treatment or by irradiation with UV light, and thus
acquire a property as a reliable etching barrier.

[0043]According to another embodiment, the etching barrier and/or the
metal contacts can be applied by means of screen-printing. The
screen-printing technology is well tried and tested industrially and is
associated with many advantages. In addition, screen-printers and the
relevant know-how are already available with many conventional production
plants, so that the plants can easily be modified for the implementation
of the method presented.

[0044]According to another aspect of the present invention, a silicon
solar cell with a selective emitter is presented. The solar cell
comprises: a solar cell substrate with a two-dimensionally extending
emitter at a front-side and/or rear-side surface thereof serving as an
emitter surface, a dielectric layer and emitter metal contacts at the
emitter surface. The two-dimensionally extending emitter has a higher
surface doping concentration in first partial zones than in adjacent
second partial zones. In other words, it is a selective emitter. The
solar cell substrate has a greater thickness in the first partial zones
than in the second partial zones. In other words, there is a small step
between the first and the second partial zones, such as can arise for
example with the back-etching of the emitter during a production process,
as has been described above. The dielectric layer, for example made of
silicon nitride or silicon oxide, which can serve for example as an
antireflection and/or passivation layer, essentially covers the whole
emitter surface and is arranged locally between the emitter metal
contacts and the solar cell substrate. In other words, the dielectric
layer separates the emitter metal contacts at least partially from the
surface of the solar cell substrate, whereby the metal contacts can
however at least locally penetrate the dielectric layer in order to
enable an electrical contact between the metal contacts and the solar
cell substrate.

[0045]The presented silicon solar cell can advantageously be produced,
amongst other things, with the aid of the method described above. On
account of the selective emitter, it exhibits a high degree of
efficiency. It is also advantageous that the dielectric layer of the
solar cell, which is unavoidable for good antireflection and passivation
properties, is located beneath the emitter metal contacts, so that the
metal contacts lie free and can be contacted or soldered without prior
removal of a dielectric layer covering them.

[0046]Several possible properties and advantages of the production method
and of the solar cell according to the embodiments of the present
invention are explained below.

[0047]The method enables the cost-effective application of a back-etched
and/or selective emitter structure with an increase in the efficiency of
the produced solar cells over four percent relative, brought about by
increasing the short-circuit current and/or open circuit voltage and/or
filling factor of the solar cell.

[0048]Apart from the increase in the efficiency, a solar cell with a
selective emitter structure can offer further advantages. In a standard
solar cell with a homogeneous emitter, the alloying of the metal contacts
can represent a critical process step. The process window for
establishing the required temperatures can be relatively small, because
the sheet resistances of the emitter of 50 to 60 ohms per square are
already at the bounds of what is possible. In the case of a selective
emitter structure, higher doping can be selected beneath the metal
contacts, i.e. in the first partial zones, so that the window of possible
process parameters is larger.

[0049]In the optimisation of thick-film pastes, a compromise has had to be
reached hitherto between finger conductivity, contact resistance and
rheology (flow behaviour). Since a good electrical contact is easier to
produce with a selective emitter, thick-film pastes can be optimised for
other parameters, such as for example a higher finger conductivity and an
optimised flow behaviour, which enables the screen-printing of finer
fingers.

[0050]The selective emitter structure can directly permit the use of
cost-effective screen-printing pastes, without the efficiency of the
solar cells being influenced unfavourably.

[0051]In addition, a selective emitter enables the appropriate use of
methods for producing finer metal contacts. For the purpose of reducing
shadow losses, it is possible to produce a metal grid with finer fingers.
Typical conventional finger widths lie in the range between 100 and 140
μm. If the finger width is reduced below 80 μm, the solar cell
delivers more current on account of the smaller shadow due to the
metallisation. The series resistance may however increase in the case of
conventional solar cells, because the contact area between the
metallisation and the substrate surface is also reduced. In the case of
the cell with a selective emitter, the specific contact resistance can be
reduced on account of the higher doping beneath the fingers, so that the
total series resistance does not increase.

[0052]In order to guarantee process stability, an additional development
of other measuring devices for use in a production line implementing the
presented production method is not required. Commercially available
optical and electrical measuring devices already used in solar cell
manufacture are capable of monitoring the process online. The process is
very easy to control and moreover is stable and variable.

[0053]Furthermore, it has emerged that, in the case of solar cells which
have been produced according to the presented methods, it is possible by
means of the back-etching--over the whole area or in the zones not
protected by the etching barrier--to produce an emitter which exhibits an
advantageous doping profile. Emitters, with which a doping agent source
has been brought directly into contact with the solar cell substrate
surface for their production and the doping agents have then been
diffused into the surface at high process temperatures, have an extremely
high doping concentration directly at the surface. This can have a
particularly unfavourable effect on the solar cell properties in the
presence of illumination with high-frequency (blue or UV) light. This
very strongly doped superficial layer can be removed by back-etching in
the second partial zones of the emitter surface, which can have a
favourable effect on the IQE in the short-wave spectral region. It has
been found that the doping profile with the back-etched emitters runs
very much flatter than in the case of emitters directly after the
diffusion, which overall exhibit the same sheet resistance. For example,
it has been found that a back-etched emitter with a sheet resistance of
60 ohms per square can, for example, have similarly good properties with
respect to the IQE and/or emitter saturation current density J0e as
a conventionally produced emitter with a sheet resistance of 100 ohms per
square. With the methods presented, relatively low sheet resistances of,
for example, 60 to 80 ohms per square can thus also be accepted in the
second partial zones which lie between the metal contacts in the finished
solar cell, without this having a negative effect on the IQE of the solar
cell. At the same time, the total series resistances inside the solar
cell can be reduced with such low sheet resistances, which overall has a
favourable effect on the efficiency of the solar cell.

[0054]It is pointed out that features described previously in respect of
individual embodiments of the present invention can be arbitrarily
combined with one another. In particular, features which have been
described for the production method can be combined with features of the
solar cell according to the invention.

[0055]The previously described and further aspects, features and
advantages of the present invention can be seen from the following
description of specific embodiments making reference to the appended
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]The invention will now be explained in further detail with reference
to the drawings.

[0057]FIG. 1 shows a cross-section of a solar cell according to an
embodiment of the present invention.

[0058]FIG. 2 (a)-(g) show a solar cell in various stages of a production
method according to a further embodiment of the present invention.

[0059]The drawings are in each case only diagrammatic sketches. In
particular, the thicknesses of the individual layers are not represented
true to scale. Identical or similar reference numbers in the drawings
denote identical or similar elements.

DETAILED DESCRIPTION

[0060]FIG. 1 shows a solar cell 1 with a p-conducting base and an
n-conducting two-dimensionally extending selective emitter 5. Emitter 5
has first partial zones 7 and, in between, second partial zones 9, the
thickness of second partial zones 9 being smaller than that of first
partial zones and the layer resistance of second partial zones 9 being
greater than that of first partial zones 7. A small step 11 extends
between first partial zones 7 and second partial zones 9. Extending over
the whole front-side surface of solar cell substrate 13, which contains
base 3 and emitter 5, is a dielectric layer 15 made of silicon nitride,
which at the same time serves as an antireflection layer and as a surface
passivation. Finger-shaped metal contacts 17 are arranged over dielectric
layer 15 in the region of first thicker partial zones 7 of selective
emitter 5, dielectric layer 15 being located between metal contacts 17
and the surface of solar cell substrate 13, but being partially
penetrated by so-called "spikes", which run from metal contacts 17 to the
surface of emitter 5 in order to produce an ohmic contact with this
surface. A two-dimensionally extending aluminium back contact 19 is
located at the rear side of the solar cell.

[0061]A sequence of production steps for the production of a solar cell
according to an embodiment of the present invention is described with the
aid of FIG. 2.

[0062]The starting point is a silicon wafer 21. This can have been
previously surface-textured and cleaned (step (a)).

[0063]A two-dimensionally extending emitter layer 5 is diffused, with the
aid of a POCl3 gas-phase diffusion at high temperatures of approx.
800 to 1000° C., into the surface of p-conducting wafer 21, which
for the most part subsequently provides base 3. With this diffusion
process, a phosphorus glass layer 23 is formed at the surface of emitter
layer 5 thus produced (step (b)).

[0064]The so-called edge insulation then takes place, in which the
electrical connection between emitter 5 produced on the front side and
the emitter region produced on the rear side is separated. For this
purpose, wafer 21 provided with the emitter can for example be subjected
to an etching plasma at its edge, so that the outermost layer of the
wafer containing emitter 5 is etched away. An etching barrier 25 is then
printed onto phosphorus glass layer 23 in the form of elongated fingers
(normal to the plane of the drawing in the figure) by means of
screen-printing. For this purpose, use can for example be made of a
screen-printing paste from the firm Peters Lackwerke (Germany) with the
name SD2052A1, which can form a layer made of an organic plastic (step
(c)).

[0065]After the material of etching barrier 25 has been hardened thermally
or by irradiation with UV light, the front-side surface of the solar cell
substrate is subjected to an HF--HNO3--H2O solution. The
etching solution first etches away phosphorus glass layer 23 in the zones
not protected by etching barrier 25 and then attacks emitter 5 lying
beneath, whereby it forms a layer 27 of porous silicon. As can readily be
seen in the enlarged detail (A), porous silicon layer 27 extends into
emitter layer 5 (step (d)).

[0066]The produced porous silicon layer is then oxidised in an acid which
contains nitric acid (HNO3) or sulphuric acid (H2SO4).

[0067]After etching barrier 25 has been removed, for example by
dissolution ("stripping") in sulphuric acid, phosphorus glass layer 23
remaining beneath and, at the same time, also the produced oxidised
porous silicon in second partial zones 9 are etched away in a
hydrofluoric acid solution (HF+H2O) (step (e)).

[0068]In this way, a selective emitter 5 with more strongly doped thick
first partial zones 7 and more weakly doped thin second partial zones 9
has thus been produced at the surface of wafer 21 serving as solar cell
substrate 13.

[0069]A dielectric layer 15 serving as an antireflection layer and
passivation layer is then deposited over the whole front-side surface,
for example in the PECVD process (Plasma Enhanced Chemical Vapour
Deposition) (step (f)).

[0071]In conclusion, it is pointed out that the terms "comprise",
"include" etc. are not intended to exclude the presence of other
additional elements. The term "a" likewise does not exclude the presence
of a plurality of elements or objects. Furthermore, in addition to the
method steps stated in the claims, further method steps may be necessary
or advantageous finally to complete the solar cell.